My two lights were new probably 2 years ago or less. I cleaned the tops this morning but didn't look dirty. The batteries failing this soon may be unusual or not... lots of web advice to change batteries every year.
It looks like tonight will be relatively warm again so I will see how successful just daylight charging works without the low temp variable. I still have low expectations for 20+ YO batteries.
another data point, one light seemed chronically less bright... I tried swapping the tops and that didn't make a difference, what did help was rotating the bottom that has the clear lens... apparently not symmetrical for light steering/output.
At dusk they switch on at slightly different times suggesting the solar cell output may not be identical as that probably dominates that (battery voltage could also be a factor).
As usual spending more time on these than they deserve... (its what nerds do).
[edit it only took two days for a weak sister to reveal itself from my decades old nicad batteries. One winked out around midnight, while the other was still lit at 4:30AM. The writing on the wall is that A) 20 yo nicads are not to be trusted, and B) I probably don't need more than 300mAh while the smallest replacements I see are typically 600mAh. [/edit]
[edit2] by process of elimination I have found the two best of my four, decades old 1000mAh nicads. Now the 2nd best one dims out around 3AM and the stronger one is still lit at 4:30AM. [/edit2]